Another Look At The Open-Source Nouveau With Fermi

Earlier this week was benchmarks
of the NVIDIA GeForce GTX 460 "Fermi" with the open-source Nouveau driver.
The reverse-engineered Nouveau support for the GeForce 400/500 series is incomplete
and requires users to generate their own custom firmware before there is even
2D/3D/video acceleration support. The initial tests on the GeForce GTX 460 also
yielded a disturbingly large performance difference between the open-source and
closed-source NVIDIA drivers, where as with previous generations of NVIDIA GPUs
the performance difference is more manageable. Here is another look at Nouveau
for Fermi, but this time from a GeForce GTX 485M.

The tests earlier this week showed the binary NVIDIA driver being
multiple times faster than the open-source Nouveau Gallium3D driver. The GeForce
GTX 460 on Nouveau even ended up being slower than graphics cards of previous
GeForce generations. It turns out that this support is due in large part due to
the Nouveau driver not yet supporting re-clocking under Fermi. The Nouveau driver
is stuck operating the graphics card at much lower core and memory clocks than
the NVIDIA driver, which dynamically adjusts the frequencies based upon load via
PowerMizer. Nouveau has begun supporting dynamic re-clocking and power management
for previous generations of NVIDIA GPUs, but nothing is in place yet for Fermi.

Because of this incomplete support, the Nouveau driver is severely
handicapped with Fermi. With Nouveau using the Linux
2.6.39 kernel, the open-source driver is running at whatever the default frequency
of the GPU and video memory were set running when the kernel module was loaded.
To force the Nouveau Fermi driver right now to run at full-speed, you would either
need to manipulate the video BIOS or first load the NVIDIA binary driver, raising
the clock speeds, and then reloading Linux (kexec -f) and then unload
the NVIDIA driver and load the Nouveau DRM without rebooting the system. Obviously,
this is far from ideal.

The current Nouveau support also lacks temperature and fan control
support for Fermi.

Fortunately, the Nouveau developers are hoping to ship support
for these critical features in the Linux 2.6.40 kernel.

In terms of the rendering problems reported earlier this week,
it turns out that it's a mysterious bug impacting the Nouveau driver that's been
referred to as "page jumping" where the virtual to physical memory address
mapping is temporarily confused.

These comments were made by Nouveau developers in this
Phoronix Forums thread. It was also mentioned that Nouveau developers have
successfully reverse-engineered NVIDIA's CUDA (Compute Unified Device Architecture)
from the binary driver. The Nouveau developers now know how to compile a CUDA
program and execute it on the hardware. However, the CUDA/GPGPU/OpenCL focus is
not as high for Nouveau developers (that's more the interest of the PCSNV fork)
so there is not code yet in the open-source Nouveau driver for actually supporting
open-source CUDA. There's also infrastructure work needed within Mesa / Gallium3D
such as by the "Clover" OpenCL state tracker too.

There is also a
CPU usage comparison and other metrics for Nouveau and the NVIDIA driver.
This is done using the system_monitor module for the Phoronix Test Suite when
setting the environmental variable MONITOR=all.

Long story short, the Nouveau GeForce 400/500 "Fermi"
support still has a ways to mature. Still needing to generate your own firmware
with there being no redistributable firmware (nor is the kernel driver able to
generate itself) and hardware acceleration not being available without it, that's
the most crippling aspect right now for most end-users. However, once you do have
acceleration support, you are with a high-performance GPU that is stuck with reduced
clock speeds as there is no dynamic re-clocking / power management support for
now. The Linux
2.6.40 kernel will hopefully address this in the coming months, which should
be an interesting release too for Intel users with more
Sandy Bridge performance improvements.

Michael Larabel is the principal author of Phoronix.com and founded the web-site in 2004 with a focus on enriching the Linux hardware experience and being the largest web-site devoted to Linux hardware reviews, particularly for products relevant to Linux gamers and enthusiasts but also commonly reviewing servers/workstations and embedded Linux devices. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics hardware drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated testing software. He can be followed via Twitter and Google+ or contacted via MichaelLarabel.com.